The GTP binding protein ras is a key protein in cell proliferation signaling pathways. Mutated forms of ras have been found in many animal and human tumor cells. One of the most important negative regulatory protein is GTPase activating protein (GAP), that catalyzes the hydrolysis of GTP in ras protein to GDP, thus keeping ras in its inactive GDP bound state. The N-terminal SH2 domain of GAP is essential for its association with ras activating receptors, and has been crystallized in the presence of a pentapeptide derived from platelet derived growth factor receptor (PDGFR). The crystals diffracted to 3.0 w on a Rigaku rotating anode X-ray source with RAXISII image plates. Preliminary experiments show that the use of synchrotron radiation will improve the quality and resolution of data collected.